1. Field of the Invention
The present invention relates to an improved process for the phosgene-free production of aliphatic and cycloaliphatic diisocyanates by the conversion of aliphatic and cycloaliphatic diamines into the corresponding biscarbamates and their continuous thermal cracking in the liquid phase without solvent, with the by-products formed during the cracking being discharged and recycled into the biscarbamate production process.
2. Discussion of the Background
One method for producing aliphatic and cycloaliphatic, hereinafter referred to as (cyclo)aliphatic, biscarbamates consists of the reaction of (cyclo)aliphatic diamines with urea and alcohols with the loss of ammonia, as described in European Patent 18 586. Other methods involve the complete or partial substitution of urea or diamines by compounds containing carbonyl groups, for example by N-unsubstituted carbamates and/or dialkyl carbonates, or mono- or disubstituted ureas or polyureas, such as those that also occur as intermediates in the above-mentioned reaction of diamines with urea and alcohols (cf. European Patents 27 952, 27 953, 28 331, and European Patent Application Disclosures 126 299 and 126 300).
The thermal cracking of (cyclo)aliphatic, and especially aromatic, mono- and biscarbamates into the corresponding isocyanates and alcohols has long been known, and can be carried out either in the gas phase at high temperatures or at relatively low temperatures in the liquid phase. However, it is a problem in both methods that unwanted side reactions also take place, in principle because of the thermal load. These side reactions not only reduce the yields, but also give rise to resinifying by-products that interfere substantially with the course of an industrial process by coating and plugging reactors and processing equipment.
Therefore, there has been no lack of suggestions for improving yields and reducing the formation of byproducts by selective chemical and process steps. Thus, catalysts are described in DE-PS 1 022 222, DE-AS 19 44 719, U.S. Pat. No. 3,919,279, and DE-AS 26 35 490 which accelerate the cracking reaction of carbamates. The disclosed catalysts are a number of basic, acidic, and organometallic compounds, that do in fact substantially improve the yields of isocyanates in comparison with uncatalyzed reactions, but are not able to prevent the formation of by-products. The same also applies to the additional use of inert solvents, to provide for the most uniform possible distribution of the supplied heat and the catalyst in the reaction medium, as recommended also in U.S. Pat. No. 3,919,279 and DE-AS 26 35 490.
It is also disclosed in European Patent 54 817 that monocarbamates can be cracked in good yields without the use of solvents at relatively low temperatures, preferably at reduced pressure, optionally in the presence of catalysts and/or stabilizers, with the cracking products, isocyanate and alcohol, being removed by distillation, by boiling the reaction mixture, and being collected separately by fractional condensation. A partial discharge of the reaction mixture to separate the by-products formed during the cracking is also described in the examples listed. The possible utilization of these residues is not disclosed.
On the other hand, in European Patent 61 013, the thermal cracking of aromatic and (cyclo)aliphatic biscarbamates is carried out with the addition of catalysts and auxiliaries comparable to those described in European Patent 54 817, again in the presence of solvents. The solvents apparently also serve to absorb nonvolatile side products that are formed, which are then separated and discarded after discharge. However, use of refluxing solvents basically leads to a reduction of the space/time yields of the isocyanates and requires an additional expenditure of energy. No information is given concerning the extent of recovery of solvent. Furthermore, auxiliaries are used that are volatile under the reaction conditions and lead to contamination of the cracking products. The high proportion of residue compared to the diisocyanate formed is also particularly noticeable and, along with the low operating pressure, casts doubt on the suitability of this method as an economical and problem-free industrial procedure.
European Patent 92 738, in part, describes the thermal cracking of the cycloaliphatic biscarbamate 5-(ethoxycarbonylamino)-1-(ethoxycarbonylaminomethyl)-1,3,3-trimethylcyclo hexane, which is fed along the inner wall of a tubular reactor in liquid form in the presence of a high-boiling solvent. Drawbacks of this process include the low yield (51.8%) and selectivity (91.9%) for the corresponding diisocyanate. Results of a continuous procedure with recycling of the recombined or partially cracked biscarbamate are not given, nor is information on the processing of the solvent containing the by-products and catalyst given.
In summary, it can be stated that in the cited publications, regardless of whether they refer only to the cracking of biscarbamates or also include their production, there are no references to a yield-improving utilization of the sometimes high proportions of residue formed during the cracking of the carbamate, which also lead to contamination of the distillate and plugging of the system components during the distillation, by decomposition and formation of resinous encrustations.
European Disclosure 133 274 describes the reaction of esters of N-substituted allophanic acids and/or polyallophanic acids with alcohols in the absence or presence of catalysts at temperatures of at least 160.degree. C. to obtain carbamates. The allophanates in this case are exclusively those that can be formed by the reaction of compounds containing isocyanate groups with compounds containing urethane groups in the distillation bottoms of the purification distillation of the crude isocyanate formed during the carbamate cracking.
However, the by-products that are formed in the cracking reactor during the thermal cracking of, in particular, biscarbamates are a mixture of a number of substances consisting of, among others, substituted high molecular weight, undistillable compounds containing uretdione, isocyanurate, allophanate, urea, polyuret, or carbodiimide groups. This is also obvious from the fact that they can be reacted only incompletely with alcohols to obtain carbamates in small proportion, like the allophanates of European Disclosure 133 274.
Thus, there remains a need for a method to prepare (cyclo)aliphatic diisocyanates by the thermal cracking of the corresponding biscarbamates by an industrially practical, economicalcirculation process, in high yields which utilizes the by-products produced in the thermal cracking and avoids the problems of the by-products coating and plugging the process equipment.